This invention relates to a telephone line interface circuit and particularly to a circuit which can be controlled by a computer or microprocessor for providing power and signal voltages to a telephone line.
Circuits of this type are used in very large numbers in telephone exchanges for providing an interface between the exchange and the telephone lines of a subscriber or group of subscribers. For this reason the circuits involved are used in very large numbers and comprise a significant proportion of the cost of a telephone exchange. Considerable effort is currently expended in developing circuits for this function which are effective and yet are cheap to manufacture.
As is well known, the telephone lines provide a TIP wire and a RING wire and the required interface circuit needs to develop voltages across these wires to communicate the necessary control functions and power voltages to the subscribers' telephone equipment.
In addition, the circuit needs to extract from the lines and communicate to the lines the analogue, or in some cases digital, signals which provide the communication from the subscribers' telephone into the exchange and thence to a remote location, for example, a further subscriber.
Microprocessor control of the voltages applied to the line for the power and control functions is necessary to make the interface circuit compatible with microprocessor controlled exchange switching circuits which are now almost universal in regard to new installations. In addition, however, the circuit must be able to accommodate the voltages which are necessary to operate the old-fashioned electromechanical switching systems and subscriber equipment which are currently installed and which will remain in service for many years to come.